An electrophotographic process visualizing an image information via an electrostatic latent image has been utilized in various fields at present and is known. The electrophotographic process is a process of generally forming an electrostatic latent image on a photoreceptor in an electrostatic charging/light exposure step, forming a toner image by developing the electrostatic latent image using a developer containing a toner in a development step, transferring the toner image onto a transfer material such as a paper, a sheet, etc., in a transfer step, and fixing the toner image onto the transfer material utilizing heat, a solvent, a pressure, etc., in a fixing step to obtain a permanent image.
In these electrophotographic processes, as a typical process of using a so-called two-component developer made of a mixture of a carrier and a toner, there is a magnetic brush process. In the process, particles having a magnetism, such as steel particles, ferrite particles, etc., are used as the carrier, a developer made of a toner and the magnetic carrier is carried by a magnet, and the developer is formed in a brush form by the magnetic field of the magnet. Then, by contacting the magnetic brush with an electrostatic latent image formed on a photoreceptor, the toner in the magnetic brush is attracted to the latent image according to the quantity of the electrostatic charge of the latent image, whereby the latent image is developed with the toner.
The carrier used in this case is largely classified into a coated carrier having a coating on the surface thereof and a non-coated carrier having no coating on the surface but because the coated carrier is excellent when considering the life, etc., of the developer, various coated carriers have been developed and practically used. As the characteristics of the coated carrier, it is required that proper electrostatic-charging properties (electrostatically charged amount, electrostatic charge distribution, etc.) can be given to the toner, the proper electrostatic-charging properties can be maintained for a long period of time. Accordingly, various kinds of coated carriers which do not change the electrostatic-charging properties of the toner, are excellent in the shock resistance and the corrosion resistance, and are stable to the environmental changes such as the changes of humidity, temperature, etc., have been proposed. In these carriers, carrier core materials are coated with a resin composition followed by curing to form the coated carriers having a relatively long life.
However, in the case of these carriers, these is a problem that the occurrence of lowering the charging properties by staining (impaction) of the carrier surface with the toner component cannot be prevented. To prevent the occurrence of the problem, it has been considered to form the coating of the carrier using a resin having a small surface energy such as the silicone resin as described, for example, in Japanese Patent Application Laid-Open No. 60-186844 (1985), etc., and the fluorine-based resin as described in Japanese Patent Application Laid-Open No. 1-13560 (1989). However, because in such carriers, the silicone resin or the fluorine-based resin described above exists only slightly in the thickness direction of the coating layer, there is a problem that when the developer is used for a long period of time, the effect of the resin is gradually lost by the abrasion or the like of the coating, and the impaction occurs again. Also, in the case of carrying out a continuous reproduction using such a developer, images excellent in the density regeneration and the image quality can be obtained in the beginning but after reproducing several tens of thousands copies, there are problems that the image density is lowered and the gradation and the graininess become poor.
Recently, a full-color copying machine has attracted attention and with the tendency, the necessity of satisfying the required characteristics unique to color copies different from black and white copies in related art has arisen. That is, most of the original of black and white copies are line images such as graphs, letters, etc., and the image area on a transfer material such as a paper is about 10% or less, while in the case of a full-color reproduction, since the original has a very large image area such as maps, photographs, pictures, etc., and further the portion having a gradation is large, a technique of faithfully reproducing them becomes necessary.
Moreover, in a digital-type electrophotographic full-color copying machine, from the requirement of obtaining a high image quality such as the halftone gradation and the graininess of a digital image, small sizing of toners has been proceeded and it is known that the preferred particle diameter of the toners is 9 .mu.m or smaller.
Accordingly, for the purpose of improving the image quality, various developers have been proposed. In Japanese Patent Application Laid-Open No. 58-129437 (1983), a non-magnetic toner is proposed. Toner particles has a mean particle diameter of from 6 to 10 .mu.m, and the particle diameters of most of the particles are from 5 to 8 .mu.m. However, because the toner particles having particle diameters of 5 .mu.m or smaller, which can clearly reproduce the fine dots of a latent image and be densely placed on the latent image, are 15% in number or less, there is a problem that the graininess and the gradation of the images formed become poor. Also, on the other hand, when the toner particles having the particle diameters of 5 .mu.m or smaller are excessive, there occurs a problem that the fluidity of the toner is reduced.
Furthermore, even when the mean particle diameter of the toner and the particle size distribution of the toner particles having the particle diameters of 5 .mu.m or smaller are proper, in the case of using a carrier in related art, good images can be obtained in the beginning, but as copying is carried out repeatedly, deterioration of images, such as the formation of fog at the non-imaged portions and the roughness of density, etc., occurs and it is difficult to repeatedly obtain the images having the required gradation and graininess. This is considered to be a phenomenon caused by that during repeating copying, only the toner which is liable to be developed is selectively consumed (called a selective phenomenon) and the toner particularly poor in the developing property remains in a developing machine.
Also, in an image pattern having a large density difference, an edge effect that only the peripheral portion of an image is emphasized occurs and an image defect that a toner does not attach to a part of the boundary of images to form a pseudo outline cannot be improved by the small-sizing of the toner alone.
Thus, in Japanese Patent Application Laid-Open Nos. 58-144839 (1983) and 61-204646 (1986), it is proposed to define the mean particle diameter and the particle size distribution of a carrier. However, these patent publications do not mention about the magnetic characteristics which greatly influence the improvement of the conveying property of the toner.
On the other hand, in Japanese Patent Application Laid-Open No. 10-2789246 (1998), the particle size distribution and the magnetic characteristics of a carrier and the particle size distribution of a toner are specifically described. However, even in the case of using the developer described in the patent publication, images excellent in the color reproducibility, the gradation, and the graininess are obtained in the beginning, but as copying is carried out repeatedly, lowering of the image quality due to the pseudo outline and the edge effect is arisen and the use of the developer is insufficient to attain the desired image quality.
Now, when continuous copying is carried out using such a developer, images excellent in the density reproducibility and the image quality can be obtained for a long period of time but a deterioration proceeds to some extent with the passage of time. In general, in the case of using a developer wherein staining of a carrier proceeds, the deterioration of image quality proceeds gradually, and hitherto, such a developer is recovered and disposed of. However, recently, environmental disruption by industrial wastes has become a problem and the regeneration of developers becomes one of the issues to resolve. With regard to the regeneration of developers, in Japanese Patent Application Laid-Open No. 47-12286 (1972), for example, a method of regenerating by heating a recovered developer at a high temperature (about 1000.degree. F.) is proposed but the high-temperature treatment may have an effect for removing the coating but has the fault that electric characteristics, etc., cannot be regenerated. Also, in Japanese Patent Application Laid-Open No. 6-149132 (1994), a method of removing the stain of a carrier without damaging the coating layer of the carrier surface by the combination with a thermally decomposable toner is proposed but the method can be applied only to the carrier using a specific coating material and is lacking in a general use.